Today's consumer electronic devices are designed toward being short, small, light and thin or with portability. This design paradigm leads to smaller speakers in all portable electronic devices. The resulting smaller physical dimension of the speakers severely limits in sound reproduction, especially in the low frequency registers, leading to consumer dissatisfaction with the sound output quality. A conventional solution to this problem is to amplify the low frequency components in the sound signal. However, this increased energy level not only leads to the extra power consumption, but also results in the speaker damage as well.
A better solution to improve the reproduction performance without boosting low frequency component is to utilize psychoacoustic techniques. Psychoacoustic technique demonstrates the existence of a phenomenon in harmonics known as ‘virtual pitch”, in which a frequency in the greatest common factor in harmonic frequencies is sensed in the brain to make people incorrectly hear sound whose frequency is close to the fundamental frequency, even if the amplitude of the fundamental frequency is zero. Thus, we can use the “virtual pitch” to make consumers feel low-frequency sound signals unable to be reproduced by speakers.
Using a full wave rectifier to generate harmonics is disclosed by the U.S. Pat. Nos. 5,668,885 and 5,771,296. A full wave rectifier and a full wave integration to generate harmonics are disclosed by a paper, “Reproducing Low-Pitched Signals through Small Loudspeakers”.
Signal clipping to generate harmonics is disclosed by the U.S. Pat. Nos. 4,150,253 and 4,700,390. Generating harmonics by a feedback loop from an output to an input is disclosed by the U.S. Pat. No. 5,930,373. A zero-crossing detector for detecting zero crossings in an input signal is disclosed by the U.S. Pat. No. 6,111,960. Modulating the input signal with at least one frequency signal to generate harmonics is disclosed by the US Patent Publication No. 2006/0159283.
The full wave rectifier is easily implemented, however it just generates even harmonics. The pitch of harmonics for the bass signals is perceived to be twice the fundamental frequency, namely, double the frequency as the original sound. This means the synthetic bass sounds an octave high to the input signal. The clipper, on the other hand, only generates odd harmonics. Previous harmonic generators also have a problem that the output spectrum envelope decay speed cannot be controlled. The speed is related to the harmonic amount that influences the perceived sound quality.
Another prior art, which uses a modified envelope detection, is disclosed by the US Patent No. 2005/0265561, and the output spectrum envelope decay speed is controlled with the parameter which is determined by comparing the input signal with the feedback signal. However, the problem to the method lies in that the harmonic envelope decay speed is not a wide range, and the phase of the output harmonics cannot be easily and arbitrarily modulated.
The above-mentioned harmonic generators also contain a drawback incapable to decide the output spectrum envelopes. If the system cut-off frequency range is high, it means the frequency range needed to be enhanced is wider. Oftentimes, the excessive or too many harmonic terms in the output actually are not necessary since the harmonic as such also influences on the higher frequency components of the original audio signal. Often, several (almost three) strong bins is needed to enhance the bass component and others are weak bins. These methods are unable to determine the output spectrum envelopes. Thus, for achieving this effect, a sharp filter must be used to filter the unnecessary harmonic components and leave the main ones. However, the sharp filters have the drawback of the high computing complexity.
It is therefore attempted by the applicant to deal with the above situation encountered in the prior art.